In spread spectrum communication systems, such as CDMA communication systems, pseudorandom noise (PN) sequences are used to generate spread spectrum signals by increasing the bandwidth (i.e., spreading) of a baseband signal. A forward link waveform transmitted by a base station to a handset may be comprised of a pilot waveform and a data waveform. Both of the waveforms are received with the same relative phase and amplitude distortions introduced by the channel. The pilot waveform is an unmodulated PN sequence that aids in the demodulation process, this is known in the art as “pilot-aided demodulation.”
Conventional pilot-aided demodulation methods typically include the steps of: demodulating the pilot waveform; estimating the relative phase and amplitude of the pilot waveform; correcting the phase of the data waveform using the estimated phase of the pilot waveform; weighting data symbols from each demodulation element in a RAKE receiver according to the estimated amplitude of the pilot waveform; and combining the weighted data symbols together. The three steps of phase correction, amplitude weighting and combining are typically performed as a “dot product” as is well known in the art. A controller having a central processing unit (CPU) and/or a digital signal processor (DSP) may perform some of the above-described steps.
Referring now to FIG. 1, there is shown a conventional CDMA receiver 100 such as those used in IS-95A or TIA/EIA-95-B compliant systems. Transmitted signals are accepted as analog information, and converted into digital I (in-phase) and Q (quadrature phase) sample stream by analog-to-digital (A/D) converter 102. A multi-finger RAKE receiver is then used to variably delay and amplify multipath delays found in the sample stream, so that degradation due to fading can be minimized. In this example, the RAKE receiver includes three demodulating fingers, demodulating finger 1 (104), demodulating finger 2 (106), and demodulating finger 3 (108) all receive the same I and Q sample stream, which has been represented as a single line for simplicity.
Each demodulating finger is assigned one of the sample stream multipath delays by a controller (not shown). PN codes and Walsh codes are generated with delays consistent with the multipath delays of the sample stream that is to be demodulated. The sample stream from the multipaths is then coherently combined in combiner 110 using a technique such as Maximal Ratio Combining (MRC). A searcher 112 processes the received samples to find the existence of a pilot signal and estimate the strength of the pilot, as well as manage information on finger status, etc.
Modem CDMA communication systems provide higher rates of data transmission through the use of code channel aggregation. CDMA receivers use a plurality of demodulators in order to demodulate the aggregated code channels. Typically one demodulator is used for each code channel that is being processed. In FIG. 2, there is shown a partial block diagram of a prior art receiver structure for one of these code channels, a typical receiver would have a plurality of these demodulator structures. In this prior art receiver section, one demodulator is required for each channel. Each of the received I and Q samples 202 (only one code channel receiving data path, I or Q, is shown to simplify the discussion) is despread and uncovered with the appropriate PN and Walsh codes in order to get a fully demodulated symbol for this particular code channel. After the fully demodulated symbol is acquired, channel estimation is performed. MRC is then preformed on the multiple diversity branches 204 or the receiver, where each of the symbols is multiplied by a weight factor that is proportional to the signal amplitude. After MRC is performed on the multiple branches, conventional descrambling, further processing and channel decoding is performed.
In FIG. 3, there is shown a partial block diagram of a prior art receiver structure like the one shown in FIG. 2, that shows how data symbols of each of sixteen aggregated code channels (W016-W1516) paths are demodulated, formed and sent to a channel decoder for further processing. While in FIG. 4, there is shown a prior art receiver section used for the demodulation of sixteen aggregated code channels (e.g., W216-W1516 are used for 1XTREME compliant systems).
As the demands on wireless CDMA handsets for more features and improved battery life keeps intensifying, a method and apparatus which can efficiently demodulate aggregate code channels by taking advantage of the inherent structure of, and relationship between, such aggregated code channels would be very beneficial in the art. It would be further beneficial if such method and apparatus would help reduce the hardware complexity and therefore the cost, power and space requirements of a CDMA receiver.